Democrats were gaining ground in the battle for House control Tuesday night, while setbacks in Indiana, Tennessee and Texas squeezed their already narrow path to a Senate majority.
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Those House races, along with several others in states where polls had closed, were being watched for insights into whether Trump can push forward with his agenda unimpeded or find himself restrained by a Democratic-controlled House.
Women also contested governor's races across the country.
In the eastern suburbs of Denver and in Kansas City and Minneapolis-area districts, GOP incumbents fell to Democratic challengers. If Bredesen wins, he'll be Tennessee's first Democrat Senator since Al Gore in the early 1990s. And Florida has a competitive US Senate election, where Democratic incumbent Bill Nelson has led Republican Governor Rick Scott by a few points. Money from both sides has poured into the race, which broke the record for the most expensive race for a House seat in history.
Almost 40 percent of voters cast their ballots to express opposition to the president, according to AP VoteCast, a national survey of the electorate, while one-in-four said they voted to express support for Trump.
Two-thirds of Americans said their vote is about Donald Trump, according to CNN exit polls, and more than half of all voters said they believe the country is on the wrong track.
During his final campaign rallies in Ohio, Indiana and Missouri, Mr Trump returned to his key campaign issues, insisting that Democrats would damage the economy and allow more illegal immigration.
He bet big on a xenophobic closing message, warning of an immigrant "invasion" that promised to spread violent crime and drugs across the nation.
The president's current job approval, set at 40 percent by Gallup, was the lowest at this point of any first-term president in the modern era.
"It's great. People are finally awake right now; they're realising that this election - the midterm election - is very important for them".
Polls show Republicans favored to gain one to three U.S. Senate seats.
Democratic Sens. Joe Manchin in West Virginia and Tammy Baldwin in Wisconsin won re-election. That is the question confronting several GOP incumbents, including New Jersey Rep. Leonard Lance. What was not normal was that Menendez had been indicted on corruption charges and escaped with hung jury.More news: SpaceX's Starman has now gone beyond Mars
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Democrats' performance in the House battlefield was mixed.
The Democratic victory, she said, "is about restoring the Constitution's checks and balances to the Trump administration", and a check on Senate Republicans.
In south Florida, former Health and Human Services Secretary Donna Shalala defeated Republican Maria Elvira Salazar.
Jared Polis (D) in Colorado became the first openly gay man elected governor, while Democrat Janet Mills was on course to become the first female governor of ME and Republican Kristi L. Noem was narrowly ahead in her attempt to become the first female governor of South Dakota. Also victorious was Republican Mitt Romney, the vanquished 2012 GOP presidential candidate, who grabbed an open Utah seat. Nearly all of the most pivotal Senate spots are unfolding in very conservative areas, such as North Dakota, Missouri, and in, where Trump won by leaps and bounds and where the Democratic incumbents could possibly give up seats to Republicans. And Massachusetts Sen. Elizabeth Warren, another potential 2020 contender, also won her race.
"In the end, we hold the House because of the strong economy", the Louisiana Republican told The Associated Press on the eve of Election Day.
Trump spent election night watching returns with family and friends at the White House, his shadow looming large over the results. Among those expected: Vice President Mike Pence and former House Speaker Newt Gingrich, an informal adviser to the president.
A Republican victory in both chambers of Congress would be a validation for Mr Trump's polarising style, a month after he solidified a conservative majority on the Supreme Court when the Senate confirmed his nominee Brett Kavanaugh after a fight over sexual misconduct accusations against the jurist.
The political and practical stakes were sky-high.
Democrats need a net gain of 23 seats to take control of the House. And they would claim subpoena power to investigate Trump's personal and professional shortcomings.
But for those wondering if a "blue wave" will increase the likelihood of a Trump impeachment, they might be surprised to know that Trump impeachment odds have held steady in regards to betting on political prediction markets.
Yet Trump's party will maintain Senate control for the next two years, at least. Democrats are relying more upon women, people of color, young people and college graduates. This victory marks a significant blow to Trump as the Democrats are now in a position to challenge the USA president's policy agenda.
The demographic divides were coloring the political landscape in different ways.
In recent years, as America has grown more deeply divided along partisan lines, this is becoming the case less and less.
In fact, on average, the party of the president loses an average of 29 House seats and 2.5 Senate seats during the midterm elections.